Process of plating by pyrolytic deposition

ABSTRACT

THERMAL DECOMPOSITION OR PYROLYSIS OF ANY KNOWN PRECURSOR COMPOUND TO DEPOSIT A PLANTING OR ADHERENT COATING OF A SOLID PRODUCT OF THE DECOMPOSITION UPON A SOLID SUBSTRATE IS ACCOMPLISHED BY FORCIBLY PROJECTING, E.G. BY PRESSURE ATOMIZATION, AGAINST THE SUBSTRATE HEATED TO THE DECOMPOSITION TEMPERATURE OF THE PRECURSOR COMPOUND, FINELY-DIVIDED PARTICLES OF A LIQUID CONTAINING THE PRECURSOR COMPOUND HAVING A PARTICLE SIZE WITHIN THE RANGE OF ABOUT 1-1000 MICRONS, WHILE THE SUBSTRATE IS MAINTAINED IN A VACUUM OR CONTINUOUS GASEOUS PHASE COMPATIBLE WITH THE SYSTEM. THE PRECURSOR COMPOUND CAN BE DISSOLVED IN AN APPROPRIATE SOLVENT OR, IF LIQUID AT THE TREATMENT CONDITIONS, CAN BE PROJECTED DIRECTLY. THE SUBSTRATE IS PREFERABLY HEATED BY INDUCTION. PREFERRED PRECURSORS ARE ORGANOMETALLIC COMPOUNDS OF METALS HAVING AN ATOMIC NUMBER FROM 4-83 WHICH LIBERATE THE FREE METAL ON DECOMPOSITION BY HEATING.   D R A W I N G

Nov. 14, 1972 J. C. WITHERS THERMOCOUPLE WIRE INDUCTION con.

OOOOlOO SUBSTRATE-/ NOZZLE -THERMOMETER OOOOOOO REACTION CHAMBER PLATING SOLUTION EXHAUST TO VACUU M PUMP -GIIOLD TRAP HOT OIL FOR CONTROL OF WALL TEMPERATURE ATOMIZING AND GMCARRIER GAS m PRESSURIZING jXL GAS INVENTOR.

' JAMES C. WITHERS ATTORNEYS United States Patent r 3,702,780 PROCESS OF PLATING BY PYROLYTIC DEPOSITION James C. Withers, Reston, Va., assignor to General Technologies Corporation Filed Feb. 11, 1969, Ser. No. 798,402 Int. C]. 1364:! 1/08 US. Cl. 117-104 R 8 Claims ABSTRACT OF THE DISCLOSURE Thermal decomposition or pyrolysis of any known precursor compound to deposit a plating or adherent coating of a solid product of the decomposition upon a solid substrate is accomplished by forcibly projecting, e.g. by pressure atomization, against the substrate heated to the decomposition temperature of the precursor compound, finely-divided particles of a liquid containing the precursor compound having a particle size within the range of about l-1()00 microns, while the substrate is maintained in a vacuum or continuous gaseous phase compatible with the system. The precursor compound can be dissolved in an appropriate solvent or, if liquid at the treatment conditions, can be projected directly. The substrate is preferably heated by induction. Preferred precursors are organometallic compounds of metals having an atomic number from 4-83 which liberate the free metal on decomposition by heating.

BACKGROUND AND PRIOR ART This invention relates to the art of plating or coating solid substrates by the thermal decomposition of a precursor compound at the surface of the substrate to be coated, a reaction product of said decomposition depositing as a solid coating on the substrate, and is con cerned with an improvement in a process of this general type according to which the substrate is contacted with a liquid precursor compound or a solution containing the precursor compound in the form of finely-divided droplets having a particle size of about 1-1000 micron projected forceably against the substrate.

Coating by pyrolytic deposition is a reasonably wellknown chemical procedure at the present time. In its classical form, the pyrolysis was carried out in a true vapor phase by vaporizing a suitable precursor of the compound to be deposited and passing such vapors, with the aid of carrier gas, if necessary, over the substrate to be coated, heated to at least the minimum decomposition temperature of the precursor, so that the precursor decomposed at the substrate surface and deposited the coating compound thereon. The characteristic feature of this process, obviously, is the exposure of the precursor to the heated substrate in a true vapor or gaseous state. An inert gas was usually maintained in the pyrolysis chamber to prevent any oxidation or other deleterious reactions. Decomposition of the precursor liberated the reaction product consisting the plating material, which deposited on the substrate as a solid continuous coating or film, usually along with gaseous by-products. US. Pat. No. 2,671,739, issued Mar. 9, 1954, is illustrative of the classical approach.

Although vapor phase pyrolytic deposition is a useful process for applying certain coatings to certain substrates and is frequently employed commercially for this purpose, is is subject to several serious disadvantages. Thus, the precursor compounds must either be available naturally in gaseous form or have a sufficient vapor pressure that they can at some reasonable temperature be vaporized and 3,702,780 Patented Nov. 14, 1972 introducuced into the reaction chamber. Precursor compounds that are sensitive to heat or lack suflicient stability to permit vaporization as well as compounds which have such low vapor pressures as to preclude vaporization under acceptable conditions are inherently disqualified for use as starting materials for deposition from a gaseous phase. Moreover, as the decomposition reaction depends on heat transferred to the vapor from the substrate, the reaction is essentially confined to the vapor-substrate interface. As molecules of the precursor compound decompose adjacent the interface, further reaction depends upon the diffusion of fresh molecules of the precursor compound from other regions of the system. Consequently, the rate of diflustion withint he vapor is a limiting factor on the rate of deposition under a given set of conditions.

In addition, the necessity for manipulating the precursor in the form of a vapor presents real handicaps. Many of the precursors commonly used for plating are hazardous to humans and elebatorate and expensive measures are hence required to seal the reaction system against leakage and to otherwise protect operating personnel.

In an effort to overcome this disadvantage, attention has been given more recently to processes for carrying out pyrolytic deposition where the substrate is contacted with a continuous liquid phase in which the precursor compound is dissolved or suspended. Examples of this approach are disclosed in US. Pat. No. 3,251,712 issued May 17, 1966, and British patent specification No. 1,025,- 897 published Apr. 14, 1966. By supplying the precursor compound in a continuous liquid phase, the difficulties inherent in the vapor phase route are indeed avoided, since the problem of thermal stability is less critical when the precursor is used in liquid form, and the vapor pressure of the precursor becomes irrelevant. Consequently, the design of the reaction system can be much simplified, and in many cases the precursor-containing liquid can even be provided in an open vessel.

The solution to the problems of vapor phase deposition by bulk liquid phase deposition, however, was found to have been achieved at the expense of other equally important factors. The first such factor is the criticality of heat transfer between a solid substrate and a continuous liquid phase. In vapor phase deposition, the vapor phase can be brought into contact with all surfaces of the substrate substantially simultaneously and instantaneously, i.e., gas injected into a chamber containing the substrate fills the chamber virtually instantaneously and thus contacts all surfaces of the substrate at essentially the same moment. A continuous liquid phase, on the other hand, lacks the same mobility and whether the liquid phase be introduced into a chamber containing the substrate or the substrate be immersed into a body of the liquid, as the above-mentioned two patents describe, a finite period of time will elapse between the times the first and last parts of the substrate contact the liquid, at least for the great majority of substrate configurations. Consequently, the different parts of the substrate remain in contact with the liquid for finitely different periods of time. The decomposition of the precursor depends for heat upon the substrate and the plating material will, therefore, deposit in varying quantities on the substrate, giving discrepancies in the thickness of the coating. The liquid can, of course, be preheated, as the art suggests, but this measure merely reduces the extent of the difiiculty rather than eliminating it. The substrate could conceivably be heated in situ within a body of the liquid, but this greatly increases the treatment time as each substrate is brought up to decomposition temperature.

In the second place, except for certain high boiling solvents, contact of the hot substrate with the liquid has been found to produce a vigorous evolution of vapor at the substrate-liquid interface. This boiling action inevitably inter feres with the rate and uniformity of the deposition, causing meager and irregular platings. Even with the high boiling solvents, the non-plating decomposition by-products are frequently gaseous and create the same problems.

Finally, the rate of deposition decreases sharply in the continuous liquid phase and only very thin coatings can be obtained within any reasonable period of time. If a preheated substrate is contacted with the liquid, the available heat is quickly absorbed into the liquid and when sufiicient heat is absorbed to reduce the interface temperature below the decomposition point, the decomposition reaction terminates. Even if the substrate is heated in situ, e.g. by induction, the liquid phase approach is poorly adapted to the formation of any except very tenuous platings. As molecules of the precursor compound decompose, fresh molecules must migrate from the surrounding liquid, and the rate of deposition is thus limited by the rate of diffusion of the precursor molecules, in the same manner as in the vapor phase technique.

THE INVENTIONNATURE AND ADVANTAGES -It has been discovered that, in substance, the advantages of both the vapor phase and liquid continuous phase approaches of the prior art can be achieved without the disadvantages of either by contacting the substrate with finely-divided particles of a liquid containing the precursor compound having a size in the range of about 1-1000 microns which are forcibly projected against the surface of the substrate to be plated, while the substrate surface is heated to a temperature sufficient to bring about decomposition of the precursor compound and is maintained within a vacuum or a gaseous atmosphere compatible with the particular precursor compound and decomposition reaction.

-By providing the precursor in the form of finely-divided particles of liquid, any precursor compound which is either naturally liquid or is soluble in a solvent can be employed without regard to its vaporization temperature, and uniform, consistent, efiicient heat transfer can be effected from the substrate to the contacting particles. The forcible projection of the liquid phase particles, as by atomization, precludes the existence of a stagnant condition at the substrate surface, as that surface is constantly bombarded with new particles from which the plating material is derived. Therefore, the rate of deposition is not diffusion limited, but can be sustained at an efiicient level for relatively long periods of time, permitting the production of coatings of increased thickness with high uniformity. An improved degree of control over the coating thickness is also made possible. The separated condition of the liquid particles contacting the substrate affords an avenue for escape of gaseous by-products without gross disruption of the uniformity of contact needed for a smooth deposition.

In general, the invention is practiced by atomizing under pressure a liquid containing the desired precursor compound against the heated substrate surrounded by an atmosphere which is compatible with the reaction system in question, the extent of atomization being within the range of 1-1000 microns indicated above.

BRIEF DESCRIPTION OF THE DRAWING In the drawing appears a schematic diagram of one embodiment of apparatus suitable for practicing the process of the present invention, the parts of the apparatus being designated by appropriate captions.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS Inasmuch as the essential improvement of the present invention resides in the condition or state in which the precursor compound is delivered to the substrate surface for decomposition thereat, rather than in the production of platings of a certain chemical nature or type or in the choice of certain classes of chemical compounds for use as precursor compounds or in the selection of particular operating conditions for the process to obtain an improved product, it follows that in all aspects of the process other than the condition or state of the precursor compound, the practice of this invention may proceed in accordance with the new well-developed knowledge and experience that exists in the art of pyrolytic decomposition.

To consider as the first of these other aspects the choice of precursor compound, any precursor compound or combination of precursor compounds known in the art which either exists directly as a liquid under normal operating conditions, or such other conditions as may be selected for a particular process, or can be dissolved in a compatible solvent under such conditions broadly qualifies for the purposes of the present invention. Obviously, a broad variety of different types or classes of precursor compounds is therefore available. Thus, virtually any derivative of any metal or metalloid element having an atomic weight from 4 to 83 which is subject to thermodecomposition at a practical temperature can, in principle, be employed here to produce a coating or plating of such metal or metalloid element as well as certain decomposition derivatives thereof. As employed herein, the term metalloid is intended to designate those elements such as silicon, boron, and arsenic, which are not metals in the strict chemical sense but behave as metals for purposes of the present invention and can be readily emyployed therein to form the desired platings achievable thereby.

To illustrate the variety of broad classes of compounds in which the precursor compounds of the present invention may fall, among the derivatives of the above-designated metals and metalloids which are used are the socalled organo-rnetallics, hydrides, carbonyls, substituted carbonyls such as nitrosyl or halide-substituted carbonyls, amines, chelates, alkyls, alkyl amines, aryls, dienyls, and acetyl acetonates. From among the numerous specific precursor compounds that could be selected from these classes, the following can be mentioned as typical:

mesitylene tungsten tricarbonyl mesitylene molybdenum tricarbonyl tungsten carbonyl molybdenum carbonyl nickel carbonyl dicumene chromium iridium trichloride bischloropentadienylpentafluorophenyl titanium trimethylamine borane trimethylsilane aluminum acetylacetonates vanadium carbonyl triisobutylalumi-num diisobutylaluminum hydride While the process of the present invention can, as has been stated, be employed with virtually any precursor compound or compounds recognized in the art as suitable for pyrolytic decomposition, it does offer special benefits or advantages for the processing of certain specific compounds within this broad category, one example of which is aluminum. The formation of aluminum coatings by vapor phase deposition has long been practiced in the laboratory with a considerable number of alkyl aluminum compounds, but the adaptation of these laboratory procedures to commercial processing has proved difficult to attain. The aluminum alkyls as a class tend to have low thermostability and thus can present serious problems when they must be handled in a vaporous condition. In addition, this class of compounds is subject to side reactions coincidental with the intended decomposition reaction, such as the polymerization of the alkyls, and these side reactions tend to result in inferior platings. By means of the present invention, aluminum coatings or platings can be formed on a volume production scale upon a variety of parts, such as aircraft fasteners, handled in batches or lengthy articles such as wire and strip products handled.

on a continuous basis. In most cases, the aluminum alkyl compound is liquid at room temperature and is subject to ready decomposition at temperatures of about 250 C. Hence, the liquid compound can simply be atomized into a chamber containing the substrate desired to be coated, heated to the preferred deposition temperature, and good quality coatings can be obtained without encountering the problems which plague vapor deposition of the same compounds.

The process of the invention also is especially beneficial in the application of platings of tungsten to substrates which tend to be poorly adherent for this compound, such as tantalum. According to published reports, attempts to deposit tungsten coatings upon such substrates using the vapor phase technique produced erratic results of generally inferior quality. Following the present invention, however, it is possible to deposit from precursor compounds such as mesitylene tungsten tricarbonyl applied as a solution in benzene, coatings that are characterized by a high degree of purity, e.g. 98% or better, and uniformly good adherence for the substrate, and to reproduce these results consistently.

Although the process of this invention is particularly well adapted for the formation of coatings or platings of pure metals or metalloids as designated above, its usefulness is by no means limited to pure platings but extends to the formation of coatings of mixtures of appropriate non-reactive but compatible compounds, as well as of certain derivatives which can be formed in the course of the thermodecomposition reaction and are of such a nature as to deposit in solid form. Notable examples of these are the carbides, nitrides, oxides, sulfides, selenides, and tellurides, such as are well-known in the art, many being disclosed or suggested by US. Pat. No. 2,671,739 referred to above. Compounds necessary to produce these derivatives can be supplied in the treatment atmosphere where it is appropriate to do so, or as additives in the plating liquid which is to be atomized into the decomposition chamber or, in some instances, as a separate liquid droplet phase interjected into that chamber. Other additives can also be incorporated into the plating liquid to serve any purpose for which they are known to be useful in prior art decomposition processes. For example, the deposition of a tungsten compound, e.g. from tungsten carbonyl, can be improved by carrying out the decomposition in the presence of an additive such as water vapor, hydrogen, or hydrogen sulfide provided in the atmosphere in the decomposition chamber or used as, or incorporated in, the gas for atomizing the plating liquid.

The nature of the substrate material is of equally little importance in the application of the present process. Any substrate that can withstand the decomposition temperature required for the particular treatment and has at least some degree of adherence for the decomposition product can ordinarily be used. Included among the classes of materials available as substrates are a broad variety of metals, both common and precious, including a number of metals which under other circumstances could serve as the plating material, as well as their various alloys and mixtures; virtually all of the ceramic materials, both natural and synthetic; as well as those synthetic resins or plastics which are sufiiciently inert. Thus, numerous commerciallyavailable steels and steel-like alloys and other alloys of iron, including stainless steel and high and low carbon steel, are useful together with many other metals, such as aluminum, magnesium, copper, titanium, nickel, cobalt, beryllium and a variety of their alloys, as well as refractory metals. In the area of ceramics can be mentioned the various glasses such as Pyrex glass, quartz, zeolite, zircon, aluminum oxide, and magnesium oxide. From among the synthetic plastics are fluorocarbon polymers such as Teflon, polyamide resins alone and reinforced with glass, epoxy resins alone and admixed with glass or graphite, polystyrene, polyethylene and polypropylene. Ma-

terials not readily falling within any of these categories can also be suitable, such as solid graphite, boron, and the like. The aforegoing lists are fragmentary and are not intended to be exclusive of many other materials.

In many instances, the precursor compound is already in liquid form at room temperature or such other atomizing temperature may be selected for it, bearing in mind that no necessity exists for the precursor compound to be heated to the actual decomposition temperature prior to coming into contact with the substrate to be plated. In such cases, the use of a solvent is not required, although even a naturally liquid precursor compound can be admixed with any solvent with which it might be miscible in order to reduce its concentration in the plating liquid or for any other purpose. Precursor compounds which are not naturally liquid, however, must be dissolved in an appropriate solvent in order to place them in a form suitable for this invention. In general, any solvent which is compatible with the precursor compound and the mechanism of the intended decomposition reaction and will dissolve sufiicient amounts of the selected precursor compound is adapted for use here. In terms of broad classes of materials, any of the aromatics, alkanes including the hydrocarbons and halogenated hydrocarbons, cycloalkanes, ethers including the alkyl ethers, glycols, polyglycols and polyaromatic ethers, fluorocarbons, alcohols, ketones, siloxides and silicones will in principle serve the purpose provided they meet the basic requirements enumerated above. Typical solvents Which have been proved to be effective include the following:

kerosene benzene and its derivatives, e.g. l-methyl-, 2-propyl-, l-ethyl-, 4-methy1-, 3-propyl-, etc.

hexane toluene and its derivatives, e.g. merthoxyxylene heptane decane nonane cyclopentane cyclohexane methylethyl ketone ethylene glycol and its derivatives, e.g. dimethyltetrahydrofuran SiCl.

TiCl

The use of the characterization compatible in the above discussion of the nature of the solvent does not, it will be appreciated, mean that the solvent will be necessarily inert with respect to the decomposition reaction that is to be practiced, although for many purposes inertness is a desirable property. As has already been suggested, for purposes of certain coatings, some other compound or compounds must be provided in order to combine with the decomposition product derived directly from the precursor and produce the desired ultimate plating material. Obviously, the solvent can contain this other compound or, where appropriate, actually serve the role of that compound. For example, if it is desired to produce a plating of a carbon or carbide derivative of a particular metal or metalloid, an organic solvent having a relatively low decomposition temperature can be selected to participate in the decomposition and deposition reaction as a carbon donor and result in the formation of the carbide or carbon derivative.

Normally, good adherence of the reaction product for the substrate will require that the substrate be free of contamination, particularly by dust, oils or grease or other material that might tend to prevent the most intimate contact between the decomposition product and the substrate surface. Thus, it is desirable, if not essential, for the substrate surface to be cleansed of all such contaminants, as well as of any superficial oxide film or the like that might impede firm deposition. Most oils and greases can be removed by any of a variety of solvents, including a number of those described above as the carrier vehicle for the precursor compound. For removing more persistent contamiants or oxide films or the like, resort may be had to chemical etching, wire brushing, vapor blasting and similar preparatory treatments.

The selection of a particular temperature for carrying out the process of this invention is dependent upon considerations which are neither crucial nor vitally affected by the novel aspects of the invention, and such selection can, accordingly, be based upon the same factors as guided the practice of the prior art procedures. Speaking from the standpoint of the broad spectrum of precursor compounds which are suitable for use here, the operating temperature can range from just over room temperature, say C., to 1200 C. Obviously, the nature of the particular precursor compound, the deposition product desired to be plated, and the nature of the substrate itself will figure in the choice of a definite operating temperature. Where platings of pure metal are the objective, account will have to be taken of the point at which the particular metal being processed is subject to thermal instability or a tendency to participate in undesirable side reactions in the presence of the by-products of the particular decomposition reaction. Reference has already been made to the possibility of forming carbon or carbide derivatives of metals as a by-product to the decomposition of precursor compounds such as the carbonyl or organo-metallic derivatives of such metals if the decomposition temperature is excessively high. On the other hand, if a plating of the metal carbon or carbide is sought, then the higher decomposition temperatures would be preferable.

In the aforegoing discussion, it will have been appreciated that the temperature in question is that at which the decomposition reaction is to take place, which will normally be the temperature of the substrate to be plated, and not the temperature at which the precursor-confining liquid is injected into the decomposition chamber. The latter temperature has no relation to the decomposition temperature except that it must obviously be less than the decomposition temperature to preclude premature reaction of the precursor compound. The injection liquid itself can be maintained at room temperature, or if preheating of that liquid is deemed advantageous in order to reduce the mass heat transfer requirements, it can be heated from any temperature above room temperature to some safe point below the actual decomposition temperature of the material involved. Obviously, the temperature of the injection liquid should be kept below the point at which the liquid begins to undergo significant volatilization.

Contrary to pyrolytic decomposition from the vapor phase, it is not necessary as a general rule in the practice of the invention to maintain the atmosphere within the decomposition chamber at any particular pressure level. Normally, operation of the process at atmospheric pressure gives entirely satisfactory results. In particular instances, it may be advantageous to increase the pressure above atmospheric in order to encourage the finelydivided particles of the injection liquid to remain in the liquid phase rather than subdivide further into what could become the equivalent of a gaseous phase upon impact with the heated substrate. Maintenance of super-atmospheric pressure in the reaction chamber may also be useful to exercise a degree of control over the crystalline growth within the plating deposited on the substrate, to increase the rate of mass heat transfer in order to improve the plating rate, and to minimize the likelihood of the solvent dissociating or otherwise breaking down into components which might possibly participate in the reaction and give undesirable by-products. Conversely, the process of this invention may be carried out under subatmospheric pressures. As is well-known in the art, certain benefits accompany deposition under reduced pressure, including an improvement in the adherence of the deposit for the substrate, the formation of deposits having a finer grain structure, and the reduction of so-called dendritic growth at the coating surface to achieve smoother coatings. As far as is known, the present process can be conducted at any reduced pressure that can be obtained by conventional evacuating equipment, say about 0.01 mm. mercury or, at the other extreme, whatever pressure the reaction equipment is designed to withstand. For relatively simple equipment of the type indicated in the drawing, the operating pressure might range as high as 10 atmospheres. For reduced pressure operation, a preferred range would be about 1-500 mm. mercury. Where reduced pressure is not advantageous, operation at normal atmospheric pressure is preferred, which could be increased up to say 2 atmospheres.

The length of time the treatment is continued is determined primarily by the thickness of plating which is desired and the rate at which the coating is formed by a given reaction. Usually a minimum of a few seconds, say 5 to 10, passes after initiation of the atomization before a perceptible layer of the plating is formed, which is the lower limit of the process. At the other extreme, one of the advantages of the invention is its capability for continuing almost indefinitely if desired. Of course, for most practical purposes, treatment for a day or two will usually sufiice. The majority of operations will fall within the range of 5-10 minutes to 4-6 hours.

The concentration of the precursor compound in the injection solution in those cases where solution is employed, has not been found to be a critical condition in the practice of this invention. In general terms, the concentration may fall within the range of 0.01-99.9% and the choice of any particular concentration within this range will be dictated by practical concerns, such as the thickness desired for the plating, the time available for such thickness to be achieved, and the like.

Where it is known by general chemical principles or from experience in the practice of pyrolytic deposition in the manner of the prior art that the presence of a catalyst may beneficially affect the rate or mode of the decomposition reaction, such catalyst may be employed for the same reasons in the practice of this invention. Illustrative results which may be accomplished or promoted by utilization of a catalyst are a decrease of the decomposition temperature to a less rigorous or more practically obtainable level, control of crystalline growth within the deposition coating and/or an improvement in the purity of the material deposited. The chemical nature or character of any particular catalyst will depend upon the circumstances within which it is to be employed and the catalyst may be oxidizing, reducing, or completely inert With respect to the decomposition reaction, dependent upon the precursor compound and the chemical mechanism by which that compound undergoes decomposition. Examples of specific catalysts that have been used with good effectiveness in the present technique are water, hydrogen peroxide, hydrogen sulfide, carbon disulfide, platinum chloride, palladium chloride, titanium tetrachloride, silicon tetrachloride, carbon monoxide, nitrous oxide, hydrogen selenide and rhodium chloride. This only partial list will suggest that compounds which for some purposes exert a catalytic effect, might for other purposes serve as the precursor compound itself. The mode of introducing the catalyst into the reaction system can vary. For example, if a particular catalyst is soluble or capable of suspension within a given precursor-containing solution or liquid precursor compound, it may be added to that solution or liquid. Alternatively, liquid or solvent soluble catalysts can be injected directly into the reaction chamber separately from the precursor-containing liquid. In certain cases, a catalyst might be provided on the surface of the substrate itself. The concentration of the catalyst within the reaction chamber may vary widely with respect to the quantity of the plating compound undergoing decomposition but, as is generally true,

very small to minute amounts of the catalyst compound will usually be effective, and excess amounts serve no particular purpose except where dictated by some peculiar circumstances.

The invention is intended to cover the use of additives other than catalysts serving, for example, to modify the chemical character or physical structure of a particular deposit or to produce a plating which is an alloy or other mixture of two or more components. Alloys or multiphase coatings other than true alloys can be obtained by the simple expedient of incorporating into a given injection solution two or more precursor compounds that decompose at approximately the same decomposition temperature to produce a compatible mixture having certain desirable properties. Moreover, the additive could take the form of finely-divided inert material dissolved or suspended in the injection liquid for co-deposition as an inert second phase Within the plating. For example, colloidal-size particles ranging from about 5000 angstrom units to microns might be obtained from an inert earth or ceramic material and included in the injection mixture in order to favorably modify the crystalline growth of a given deposit to produce a very fine-grained, randomlyoriented crystalline array. Refractory metal particles, inert at the particular treatment temperature, might be included to produce a coating which is denser, harder, or more resistant to heat or flame.

Considerable experience in the practice of this invention indicates that the particle size of the finely-divided particles or droplets of the precursor-containing liquid injected into the decomposition chamber may vary within the range of approximately 1-1000 microns. It is preferred that the size of the particles be such that the precursor-containing liquid exists within the decomposition chamber as a dispersed phase having the appearance of a fog or fine mist inasmuch as a particularly good coating action has been achieved under such conditions. A preferred range of particle size consistent with the juststated condition is the range of about 10-100 microns. In general, the finer the spray, the greater the realization of the benefits which are inherent in this invention. Very large droplets, that is, droplets much exceeding the maximum of 1000 microns specified above, have not proved to be elfective and tend to be subject to the same kinds of difficulties that characterize the bulk liquid deposition process of the prior art. The rate of delivery of the injection liquid will vary widely with varying operating conditions, being directly affected by the size of the decomposition chamber as well as the rate of consumption of the liquid which would depend upon the area of the surface being plated and, in the case of the plating of continuous rods, strips and the like, the rate at which the substrate was fed through the chamber. Accordingly, it has not been practically possible to ascertain the broadest limits for this variable. As a general rule, it can be stated that the rate should not be so high as to result in the collection within the system of significant amounts of bulk liquid. In other words, the rate at which fresh liquid is injected into the system should be approximately consistent with the aggregate of the rate of consumption of the liquid in the decomposition reaction and the rate at which the particles of unreacted liquid are evacuated from the decomposition chamber. For a chamber having a capacity of approximately two cubic feet, good results have been ob tained with a delivery rate in the range of about 01-10 gallons per hour of operation.

Except for those particular treatments in which the presence of atmospheric air is needed as the atmosphere for the decomposition in order to achieve some particular type of reaction product, the reaction system should in advance of processing be purged of atmospheric air and filled with some more appropriate gas to serve as the reaction atmosphere. This purging step is conventionally practiced as a preparatory step to vapor phase deposition in the prior art using an appropriate inert gas fed into the system in sufficient quantities to expel all of the original atmosphere therefrom and replace it with the inert gas. The same purging technique can be employed in accordance with the invention, if desired, but a more economical way of accomplishing this step is to flush the system with a solvent, preferably with the same solvent that is used as the carrier for the precursor compound where the same is processed in the form of a solution. Thus, the reaction chamber and the various lines feeding thereto are simply filled with the solvent while vented to the atmosphere, and then this liquid is drained, the vapors evaporating from it and filling the system as an. inert gas. If the solvent is relatively non-volatile, some other inert gas can be fed into the system as the liquid solvent is drained therefrom. Other modes of purging could likewise be employed if preferred.

DESCRIPTION OF ILLUSTRATED APPARATUS One embodiment of apparatus which has been found well-adapted to the needs of this invention is illustrated in schematic fashion in the accompanying drawing which will now be described. According to this embodiment, the reaction chamber takes the form of a generally cylindrical hollow-walled tube arranged on a vertical axis, hot oil being circulated within the interior of the hollow wall in order to control the temperature thereof. At the lower end of this chamber is an atomizing nozzle while at the opposite end is an exhaust port feeding to the suction side of a vacuum pump through a condensation trap. Alternatively, the exhausted vapor may be recycled for further use in the system by means not shown. Within the chamber is disposed the substrate to be plated, represented in the drawing by a rectangular plate-like structure, and this substrate is connected to the leads of a thermocouple which pass out of the chamber to a meter in order to permit the temperature of the substrate to be directly determined and thus controlled. Surrounding the chamber in the region occupied by the substrate is an induction coil which can be actuated to heat the substrate to whatever temperature has been selected for the decomposition. The atomizing nozzle is supplied with the solution to be injected into the system, herein labeled plating solution, from a container thereof, the interior of which is pressurized by a pressurizing gas preferably inert in nature. Atomization in the particular nozzle illustrated in the drawing is accomplished with an atomizing gas which is compatible with the precursor compound and the decomposition reaction as that term has already been defined. The atomization is continued for that period of time necessary to produce the desired thickness of plating on the substrate which period will, of course, depend upon the concentration of the precursor compound, the rate of delivery into the chamber and like variables.

Although an atomizing nozzle of the type using as pressurized carrier or atomizing gas is described above, different modes of atomization are equally useful. Thus, the plating liquid could be projected by direct pressure through a spray head of proper design or a dispersed phase of the liquid could be formed outside the reaction chamber and flowed into the chamber against the substrate.

The following specific examples will serve to further explain and illustrate the practice of the invention.

Example 1 A section of tantalum metal to be plated was cleaned of all surface contamination by vapor blasting and placed within the deposition chamber of the just-described apparatus. Heating by induction was then begun and continned until the section was heated to 500 C. as determined with a thermocouple. A 1% solution of mesitylene tungsten tricarbonyl in benzene was then pressure atomized in the form of particles having a size of about microns into the chamber generally against the substrate surface. During the atomization, the chamber was filled with benzene vapor maintained at atmospheric pressure. Atomization was continued for a period of 10 minutes. When the substrate section was removed from the chamber and examined, it was found to have a smooth, silvery coating of brownish-black color. This coating had good adherence to the substrate and was measured at a thickness of 0.1 mil. By spectrographic analysis, its composition was found to be 98% tungsten.

Example 2 The substrate in this example was a sample of aluminum prepared for plating by degreasing with trichlorethylene, superficial etching in an alkaline commercial aluminum etch, followed by rinsing in water and drying with a blast of warm air. This sample was placed in the reaction chamber which was filled with heptane vapor pressurized to 10 p.s.i. and the sample heated to 400 C. by infrared radiation from a radiant heater stiuated within the chamber. A 10% solution of tungsten hexacarbonyl in heptane was injected into the chamber using a venturi atomizing nozzle giving an average particle size of 10 microns. Injection was continued for 30 minutes after which the sample was removed and examined. It was found that a coating of 2 mil thickness and blue-black color had been produced which tested to a Vickers hardness of 2200. Inasmuch as pure tungsten is known to have a hardness on the same scale within the range of 6001200, the above test value indicated that the coating contained tungsten carbide.

Example 3 A low-carbon steel sample was cleaned by sand blasting, introduced into the reaction chamber, which was purged with titanium tetrachloride, and heated to 650 C. by induction. A 3% solution of tungsten hexacarbonyl dissolved in titanium tetrachloride was pressure atomized as particles having a size of 25 microns into the chamber while the same was maintained under a reduced pressure of 200 mm. mercury. Atomization was continued for 2 hours after which the sample was removed and observed. A coating had been formed thereon which was smooth, shiny and gray-black in color with good adherency to the substrate. Its thickness was measured at 10 mils and its Vickers hardness at 875, from which its composition was deduced to be relatively high purity tungsten.

Example 4 A copper sample cleaned by chemical etching was heated to 600 C. by conduction within the reaction chamber, which had been purged with benzene. A 1% solution of tungsten hexacarbonyl in anhydrous benzene to which had been added one-tenth of a percent of water as a catalyst, was atomized with a particle size of 200 microns into the chamber for one hour at a reduced pressure of 100 mm. mercury. Examination of the sample indicated that a smooth, shiny blue-black coating had resulted with a thickness of 1.5 mil and a hardness of 800 by the Vickers scale.

Example 5 A sample of aluminum oxide was heated by conduction tion 550 C. under atmospheric pressure in an atmosphere of cyclopentane. A 3% solution of molybdenum pentacarbonyl dissolved in cyclopentane containing 1% water as a catalyst, was atomized at a particle size of 500 microns within this chamber under pressure for 20 minutes after which the sample was examined. A bright gray coating had been obtained with a thickness of 0.5 mils and a hardness of 900 Vickers.

Example 6 A 10% solution of nickel carbonyl in toluene was venturi atomized at a particle size of 1000 microns over a graphite substrate heated to 125 C. by radiation while maintaining the reaction chamber under atmospheric pressure and with a toluene atmosphere. After atomization 12 for a period of 40 minutes, the substrate was examined and found to have a smooth coating of shiny nickel color with a thickness of 5 mil.

Example 7 A 50% solution of nickel carbonyl in a mixture of xylene and 5% water was pressure atomized to give particles of 50 microns size for a period of 10 minutes over a nylon sample heated by radiation to a temperature of 100 C. under a reduced pressure of 300 mm. mercury. On examination, the sample was found to bear a shiny nickel coating of 0.1 mil thickness which was adherent even when the sample was cut with a saw. The nickel coated nylon was then given a further coating of aluminum by means of electrodeposition.

Example 8 Example 7 was repeated except that the water was replaced with a like amount of silicon tetrachloride as a catalyst, and like results were obtained.

Example 9 Steel nuts and bolts, cleaned by vapor blasting, were heated to 260 C. by induction while tumbling within a plastic barrel rotating within the decomposition chamber filled with nitrogen vapor. Concentrated diisobutyl aluminum hydride was pressure atomized at a particle size of microns into the barrel for a period of five minutes while the system was maintained at atmospheric pressure. The treated nuts and bolts were found to have a shiny aluminum coating of 0.3 mils thickness and a purity of 99.99% according to spectrographic analysis. The thickness of the coating was also found to be highly uniform on the inside as well as the outside of the nuts, even at the top and bottom of the threads.

Example 10 Example 9 was repeated using a 50% solution of diethyl aluminum hydride in kerosene atomized over titanium nuts and bolts and substantially the same results were obtained as in Example 9 with steel nuts and bolts.

Example 11 Concentrated triisobutyl aluminum was spray atomized at a particle size of 150 microns onto a beryllium wire substrate, resistance heated to 280 C., and continually passed through the decomposition chamber filled with argon vapor. The chamber was maintained under atmospheric pressure, and the feed rate of the wire was such that each section thereof remained within the chamber for approximately three minutes. The plated wire was found to have a highly adherent one mill coating of aluminum thereon, the adherence being sufficiently strong that the wire could be broken before rupture of the coating occurred.

Example 12 A foil of sheet tungsten heated by electrical resistance to 600 C. was fed continuously through the decomposition chamber maintained under a reduced atmosphere of mm. mercury, with a treatment atmosphere of carbon monoxide. A 1% solution of iridium trichloride in isopropyl alcohol was pressure atomized in particles having a size of 40 microns over the foil, which was fed at a rate giving a retention time within the chamber of approximately one miuute. The treated foil was found to have a 0.01 mil thickness coating of very shiny iridium metal.

Example 13 A 1% solution of diborane in tetrahydrofuran was pressure atomized with a particle size of 50 microns over a length of carbon fiber heated by electrical resistance to 600 C. within a chamber maintained under a reduced atmosphere of 10 mm. mercury. The atomization was continued for ten minutes and on examination of the 13 treated fiber, it was found to have a lustrous boron coating 2 mil in thickness.

Example 14 A solution of titanium hydride in titanium tetrachloride was pressure atomized with a particle size of 10 microns for ten minutes over a substrate of 4130 steel heated inductively to 800 C. within a chamber maintained at atmospheric pressure and filled with titanium tetrachloride vapor. A highly adherent titanium coating with a thickness of 0.2 mil was obtained.

Example 15 A 1% solution of bischloropentadienyl bispentafluorophenyl titanium in benzene was pressure atomized in particles sized at -100 over a steel sample inductively heated to 600 C. for a period of 20 minutes under atmospheric pressure in a benzene atmosphere. The substrate was found to have a 0.3 mil thick lustrous gray coating which by analysis was found to be composed of approximately titanium and titanium carbide.

Example 16 A 3% solution of tetraethoxysilane in toluene was pressure atomized in particles sized 100 microns for 20 minutes over a tungsten substrate heated inductively to 500 C. and maintained at atmospheric pressure in a toluene atmosphere. The result was a milky white coating of silicon dioxide having a thickness of 0.1 mil and good adherency for the tungsten substrate.

Example 17 A benzene solution containing 3% nickel carbonyl, 3% carbon disulfide and 3% colloidal silica, was pressure atomized in particles sized microns for a period of one hour over a substrate of styrene plastic heated by radiation to C. and maintained under atmospheric pressure in a benzene atmosphere. The result was a finegrained coating of 10 mil thickness which on microscopic analysis was found to contain a dispersion of silica particles at the boundaries of crystalline grains of nickel within the deposit.

What is claimed is:

1. In a method of coating a solid substrate by pyrolytic deposition in which a precursor compound of a heat labile organic derivative of a metal or metalloid having an atomic weight of 4 to 83 capable of decomposing at a given temperature into a free metal or metalloid reaction product adhering to said substrate, is contacted with said substrate, said substrate being preheated to at least the decomposition temperature of said precursor compound, the improvement comprising the step of forcibly projecting against said preheated substrate an atomized spray of finely-divided particles of a liquid containing said precursor compound, said liquid particles having a diameter in the range of about 1-1000 microns, the substrate being maintained at at least said decomposition temperature for a time sufiicient to form said coating and within a vacuum or a gaseous atmosphere compatible with the decomposition reaction.

2. The method of claim 1 wherein said liquid comprises a solution of said precursor compound in a solvent therefore, compatible with the decomposition reaction.

3. The method of claim 1 wherein said precursor compound is liquid and is projected directly against said substrate.

4. The method of claim 2 wherein said solution is atomized into an atmosphere consisting essentially of vapor of said solvent.

5. The method of claim 1 wherein said substrate is maintained under a vacuum of about 1 to 500 mm. mercury.

6. The method of claim 1 wherein said substrate is maintained under a pressure of one to ten atmospheres.

7. The method of claim 1 wherein said substrate is maintained at a temperature within the range of about 2S1200 C.

8. The method of claim 1 wherein said liquid particles are in the size range of from 10 to microns.

References Cited UNITED STATES PATENTS 2,700,365 11/1955 Pawlyk 117104 X 3,462,288 8/ 1969 Schmidt et al. 117107.2 X 2,770,558 11/1956 Gaiser 117--35 S 3,375,129 3/1968 Carley et al. 117130 R X 3,375,149 3/1968 Kolodney 117130 R X 3,432,331 3/1969 Braddy et a1 117-104 X 3,446,652 5/1969 Smith 117104 3,464,844 9/1969 Williams 117130 R X 3,498,825 3/1970 Wiens 117-104 X OTHER REFERENCE.

Article, Chemical Engineers Handbook by John H. Perry et al., 4th edition, copyright 1963 by McGraw-I-Iill Book Co. Inc.

EDWARD G. WHITBY, Primary Examiner US. Cl. X.R.

117--107.2 R, 119, R, 135.1, R 

